The term "formose" in the context of the present invention means the known mixtures of low molecular weight polyhydroxyl compounds (polyhydric alcohols, hydroxy aldehydes and hydroxy ketones) which are produced by the condensation of formaldehyde hydrate.
The preparation of mixtures of polyhydric alcohols, hydroxy aldehydes and hydroxy ketones by the auto-condensation of formaldehyde hydrate has been described in the literature. Examples, include Butlerow and Loew, Annalen 120, 295 (1861); J.pr. Chem. 33,321 (1886); Pfeil, chemische Berichte 84, 229 (1951); Pfeil and Schroth, chemische Berichte 85, 303 (1952); R. D. Partridge and A. H. Weiss, Carbohydrate Research 24, 29-44 (1972); the formoses of glyceraldehyde and dihydroxy acetone according to Emil Fischer; German Pat. Nos. 822,385; 830,951 and 884,794; U.S. Pat. Nos. 2,224,910; 2,269,935 and 2,272,378 and British Pat. No. 513,708. These prior art processes have certain disadvantages (poor volume/time yields and colored by-products). New processes have recently been developed by which virtually colorless formoses which are free from undesirable by-products may be prepared in high yields using the conventional catalysts.
According to one of these new processes, the condensation of formaldehyde hydrate is carried out in the presence of catalysts consisting of soluble or insoluble lead (II) salts or of lead (II) ions attached to high molecular weight carriers and in the presence of a co-catalyst consisting of a mixture of hydroxy aldehydes and hydroxy ketones which may be obtained from the condensation of formaldehyde hydrate and which is characterized by the following molar ratios:
Compounds having 3 carbon atoms/compounds having 4 carbon atoms: from 0.5:1 to 2.0:1
Compounds having 4 carbon atoms/compounds having 5 carbon atoms: from 0.2:1 to 2.0:1
Compounds having 5 carbon atoms/compounds having 6 carbon atoms: from 0.5:1 to 5.0:1. The proportion of components having from 3 to 6 carbon atoms is at least 75%, by weight, preferably more than 85%, by weight, based on the total quantity of co-catalyst.
The reaction temperature is generally from 70.degree. to 110.degree. C., preferably from 80.degree. to 100.degree. C. The pH of the reaction solution is adjusted by controlled addition of an inorganic or organic base, first to a value of from 6.0 to 8.0, preferably from 6.5 to 7.0, until from 10 to 60%, preferably from 30 to 50%, of the starting material has been converted. Thereafter the pH is adjusted to a value of from 4.0 to 6.0, preferably from 5.0 to 6.0. It is surprisingly found that the ratios of products in the resulting mixtures of polyols, hydroxy aldehydes and hydroxy ketones may be varied in a reproducible manner by this particular control of the pH followed by cooling at different residual formaldehyde contents (from 0 to 10%, by weight, preferably from 0.5 to 6%, by weight).
The auto-condensation of the formaldehyde hydrate is stopped by cooling and/or by inactivation of the lead-containing catalyst by means of acids. The catalyst is then removed and, if desired, water contained in the products is removed by distillation. Details of this procedure may be found in German Offenlegungsschrift No. 2,639,084.
Another possibility of preparing highly concentrated, colorless formoses in high volume/time yields consists of condensing aqueous formalin solutions and/or paraformaldehyde dispersions in the presence of a soluble or insoluble metal catalyst and in the presence of a co-catalyst which has been prepared by partial oxidation of a dihydric or polyhydric alcohol which has a molecular weight of from 62 to 242 and contains two adjacent hydroxyl groups or a mixture of such alcohols. The pH of the reaction solution is adjusted during condensation by controlled addition of a base. The pH is first maintained at from 6.0 to 9.0 up to from 5 to 40% conversion of the starting material and is thereafter adjusted to from 4.5 to 8.0 until the condensation reaction is stopped. In this second stage, the pH is from 1.0 to 2.0 units lower than in the first reaction phase. The reaction is then stopped at a residual formaldehyde content of from 0 to 10% by weight by inactivation of the catalyst and the catalyst is removed. Details of this process can be found in German Offenlegungsschrift No. 2,718,084.
High quality formoses may also be prepared by the condensation of formaldehyde in the presence of a metal catalyst and more than 10%, by weight, based on the formaldehyde, of one or more dihydric or polyhydric low molecular weight alcohols and/or higher molecular weight polyhydroxyl compounds. Details of this process can be found in German Offenlegungsschrift No. 2,714,104.
It is particularly economical to prepare formose directly from formaldehyde-containing synthesis gases. i.e. without first obtaining aqueous formalin solutions or paraformaldehyde. The synthesis gases obtained from the large scale industrial production of formaldehyde are conducted continuously or discontinuously at temperatures of from 10.degree. to 150.degree. C. into an absorption liquid consisting of water, monohydric or polyhydric low molecular weight alcohols and/or higher molecular weight polyhydroxyl compounds and/or compounds capable of ene-diol formation as co-catalysts and/or, as catalysts, soluble or insoluble metal compounds optionally attached to high molecular weight carriers. The absorption liquid is maintained at a pH of from 3 to 10. The formaldehyde is directly condensed in situ in the absorption liquid (optionally also in a following reaction tube or a following cascade of stirrer vessels). The auto-condensation of formaldehyde is stopped at a residual formaldehyde content of from 0 to 10%, by weight, in the reaction mixture by cooling and/or inactivation of the catalyst with acids. The catalyst is then finally removed. Details of this process can be found in German Offenlegungsschriften Nos. 2,721,093 and 2,721,186.
For most of the processes described above, divalent lead ions are the preferred catalyst. In the presence of compounds of divalent lead the auto-condensation of formaldehyde hydrate will proceed both at a neutral and a slightly acid pH in a high volume/time yield and substantially without undesirable side reactions. For some applications for formose (e.g. when it is to be used as substrate for micro-organisms or before the catalytic hydrogenation to polyhydric alcohols), it is necessary to remove the lead ions present in the products of the process. An obvious method for removal is by chemical precipitation (for example by the addition of sulphuric acid, sodium sulphate, sodium carbonate, sodium sulphide or carbon dioxide gas under pressure). It is found, however, that the hydroxy aldehydes and hydroxy ketones contained in formose have an exceptionally powerful capacity to form complexes with metal ions. In a production setting, removal of lead from aqueous formose solutions by chemical precipitation is too incomplete or is relatively expensive. In addition, formose solutions are difficult to separate from the precipitated lead salts by filtration. Moreover, for economical reasons, the lead salt would have to be converted into a soluble form so that it could be recycled for catalysing the formation of formose. This entails additional expenditure.
Another possibility lies in the removal of lead ions from formose solutions by means of ion exchange resins. The disadvantage of this method, however, is that in practice very large quantities of exchange resin would have to be used. Rinsing and regenerating liquid would also be needed, which would make the removal of the total quantity of lead from formose by means of ion exchange resins technically too complicated and expensive.